In recent years, because of growing interest about environmental problems, aqueous coatings and powder coatings have attracted attention as low-pollution coatings free from organic solvents. Also in such coating systems, metallic finery is required in various applications like the conventional organic solvent-based coatings. Use of metal pigments is therefore indispensable. In powder coating, however, objects to be coated are limited substantially only to metals because of restrictions due to electrostatic coating and high temperature baking. In the case of aqueous coatings, there may be problems that a metal pigment reacts with water in the coating to turn black or generate a hydrogen gas. A current problem is how to secure the storage stability of coatings.
In order to solve a problem about storage stability in use as an aqueous coating, i.e., a problem about water resistance, some technologies have already been developed, for example: a method in which a metal pigment is treated with a phosphoric acid-based or phosphate-based additive (Japanese Patent Laying-Open No. 63-054475 (Patent document 1), Japanese Patent Laying-Open No. 61-47771 (Patent document 2), and Japanese Patent Laying-Open No. 7-133440 (Patent document 3)), a method in which a metal pigment is treated with Mo (molybdenum) compound (Japanese Patent Laying-Open No. 6-057171 (Patent document 4)), and a method in which a metal pigment is coated with an oxide film such as silica (Japanese Patent Laying-Open No. 2003-41150 (Patent document 5), Japanese Patent Laying-Open No. 2004-131542 (Patent document 6), and Japanese Patent Laying-Open No. 2004-124069 (Patent document 7)).
On the other hand, from a viewpoint of cost, for example in plastic coating for cellular phones, personal computers and the like, one-layer coating is required. In this case, excellent chemical resistance is also required. As an approach for improving the chemical resistance, aluminum pigments coated with a resin (Japanese Patent Laying-Open No. 62-253668 (Patent document 8), Japanese Patent Laying-Open No. 64-40566 (Patent document 9)), and the like have been developed. However, although organic solvent-based coatings can satisfy the market demand, there is a problem of poor water resistance when being used as an aqueous coating.
In general, there is a tendency that it is difficult to satisfactorily impart chemical resistance in surface treatment for imparting water resistance and it is difficult to satisfactorily impart water resistance in surface treatment for imparting chemical resistance. Therefore, no technologies of reconciling water resistance and chemical resistance at practically satisfactory levels have been established. Although there are some attempts to satisfy both water resistance and chemical resistance at a high level by combined use of a technology of imparting storage stability in aqueous coatings and a resin coating technique (U.S. Pat. No. 2,885,366 specification (Patent document 10), Japanese Patent Laying-Open No. 1-129070 (Patent document 11), Japanese Patent Laying-Open No. 7-3185 (Patent document 12), and Japanese Patent Laying-Open No. 2002-121423 (Patent document 13)), none of them has reached a practically satisfactory level.    Patent document 1: Japanese Patent Laying-Open No. 63-054475    Patent document 2: Japanese Patent Laying-Open No. 61-47771    Patent document 3: Japanese Patent Laying-Open No. 7-133440    Patent document 4: Japanese Patent Laying-Open No. 6-057171    Patent document 5: Japanese Patent Laying-Open No. 2003-41150    Patent document 6: Japanese Patent Laying-Open No. 2004-131542    Patent document 7: Japanese Patent Laying-Open No. 2004-124069    Patent document 8: Japanese Patent Laying-Open No. 62-253668    Patent document 9: Japanese Patent Laying-Open No. 64-40566    Patent document 10: U.S. Pat. No. 2,885,366 specification    Patent document 11: Japanese Patent Laying-Open No. 1-129070    Patent document 12: Japanese Patent Laying-Open No. 7-3185    Patent document 13: Japanese Patent Laying-Open No. 2002-121423